In applications such as mobile/remote asset monitoring/tracking using GPS or like global positioning systems, it is desirable that the antenna be as omnidirectional as possible, providing sufficient gain for reliable system operation down to very low elevation angles. For marine applications in particular, operation down to negative elevation angles is desirable to account for operation in northern latitudes in high seas.
Furthermore, most satellite communication systems see substantially improved capacity/efficiency and reliability when the amplitude spread between inbound, also known as uplink or return, link signals is reduced. CDMA systems can detect more simultaneous carriers. TDMA systems can more reliably detect collisions. FDMA systems can avoid interchannel interference.
While a number of factors contribute to amplitude spread, such as multipath and satellite beam contours variation, a substantial portion of amplitude spread is related to mobile terminal antenna gain variation over azimuth and elevation angle. Elevation angle variation is typically larger than azimuth variation. Therefore, it is desirable for the radiation pattern to be as uniform as possible over solid angle of interest.
Antenna structures typically used for these applications include crossed dipole and Quadrifilar helix antennas. Both of these structures are circularly polarized antennas, which is a requirement for mobile satellite communications. The degree of circular polarization is defined in terms of the axial ratio, which is the ratio of orthogonal components of the electric field. For a fully circularly polarized antenna, which is desirable in a GPS application, both components are of equal magnitude, and the axial ratio is therefore unity.
The difficulty with these structures is that they require significant height to achieve the low elevation angle performance required by some systems. For example, a quadrifilar helix antenna mounted on a 20 cm ground plane requires a 10 cm height helix achieve −2 dBic at 5 degrees elevation angle. Variation between boresight and 5 degree elevation angle is 5 dB.
While a quadrifilar helix antenna is symmetrical and does not require a ground plane, in practice a ground plane is present because of the need to provide electronic circuitry in the same housing as the antenna. The printed circuit board mounting the electronic circuitry provides the ground plane. While cost effective, this level of integration due to the presence of a ground plane is a limiting factor in performance.
The ground plane inhibits operation at low elevation angles because it blocks/interferes with the radiation from the antenna. The radiation pattern at low elevation angles is of interest because if, for example, the antenna is mounted on a ship, the ship will roll from side to side, and the ground plane can tilt several degrees. In order to pick up a satellite close to the horizon, the antenna needs to be able to respond to signals at angles below the ground plane. Moreover, it is important to maintain an axial ratio as close to unity as possible in order to maintain circular polarization.
In the paper entitled “Method for Broadening the Beamwidth of Crossed Dipoles for GPS Applications”, Wei et al. Progress in Electromagnetics Research Letter, Vol. 12, 31-40, the authors propose the use of parasitic strips around the crossed dipole antenna for the purposes of broadening the beam. While the beam is broadened at some frequencies, this proposal does reduce the required height to achieve a given level of performance. The antenna disclosed is 8 cm tall and exhibits approximately 8 dB of variation between boresight and 5 degree elevation angle. This is substantially worse than the 5 dB of the quadrifilar antenna referred to above.
A number of factors contribute to the relatively poor performance of this configuration. Firstly, the crossed-dipole configuration itself is quite directive which implies that a lot of improvement is required by the monopoles to achieve the desired level of performance. Secondly, because of the height of the crossed dipole relative to parasitic strips, the amount of radiation that they can influence is limited. If one tried to lower the cross-dipole antenna in order to promote coupling to the parasitic strips, this would lead to reduced low elevation performance due to ground plane interference/blockage. Moreover, the amount by which the height can be lowered is limited due to the requirement that the dipole extend nominally λ/4 above the ground plane.